Refrigeration systems, such as used in supermarkets for cooling food storage fixtures, contain a compressor system having one or more compressors for compressing a refrigerant fluid. Refrigeration compressors must be lubricated for proper operation. Some amount of compressor lubricating oil inevitably is pumped over with the compressed refrigerant, circulates through the refrigeration system, and returns to the compressor crankcases. Oil is pumped over in both reciprocating and rotary compressors, due to blow-by of oil from the crankcase around the pistons in a reciprocating compressor or the vanes in a centrifugal compressor. As a compressor wears in service, clearances between moving parts increase, and greater amounts of oil are pumped over. As more oil is pumped over by a compressor, more oil must be added to its crankcase to maintain the oil level at its proper level. Excessive blow-by indicates compressor wear, and leads to excessive amounts of pumped over oil circulating in the refrigeration system. This adversely impacts refrigerating capacity and efficiency, and can reduce the compressor oil level below that necessary for adequate lubrication.
When multiple compressors are connected in parallel to a common suction line, the lubricating oil returning through the suction line will not evenly distribute itself among the several compressors. Further, the amount of oil pumped over by any one compressor will be different from that of the other compressors. It is therefore important to monitor the oil level of each individual compressor, and to maintain the crankcase oil level for each compressor within acceptable limits. Oil separators are typically placed in the common compressor discharge line. The separated oil is typically returned to a reservoir and then metered back to the compressors through individual float valves which detect a low compressor oil level. However, these systems do not measure the flow rate of oil added to the compressors.
Previous attempts to overcome these problems have included interconnecting the crankcases of multiple compressors so that the crankcase oil levels equalize. However, this response does not address the problem of ensuring adequate lubrication when oil levels fall below acceptable limits, such as may occur when large quantities of oil are pumped over due to compressor wear. Further, this solution cannot be used unless the pressures are equal in all the interconnected crankcases and the compressors are all at the same height on their foundations. The present invention overcomes the problems of the prior art.